This invention relates to spiral wound electrolytic cells, and methods of winding the same, and especially where high stack pressure of the cell pack is required.
It has long been recognized that cells of the spirally wound or so-called jelly roll type offer advantages of high power density and ease of automatic assembly. It is also known that certain types of rechargeable alkaline cells require firm stack pressure among the components of the cell pack to improve the life of the cell, particularly in cells where dendrite growths cause internal short circuiting. To obtain this relatively high stack pressure it has been the common practice to use slotted or collapsible mandrels having fairly large diameters to accommodate the torques encountered during winding. When smaller diameter mandrels have been used, it has previously been necessary to employ driven pressure rollers to obtain the torque -- with the result that the cell pack is subjected to an abrasive circumferential force. Neither of these methods have been satisfactory for the so-called mini-type cells, e.g., the AA size which cannot be wound with such large mandrels or be subjected to such abrasive forces and still provide suitable mechanical integrity, energy densities and discharge capacities. The most suitable mandrel for this type of cell is one of the driven type that will withstand the torques supplied and yet leave only a small axial void (relative to the diameter of the cell) when the cell pack is removed from the mandrel.
While as previously mentioned it has been advantageous to minimize the volume occupied by the axial void found in the cell pack, some void is necessary for recombination of evolved gases within the cell. There is a tendency for this void to become drastically reduced in size or to diminish completely when eloctrolyte is initially added to the cell and the cell pack undergoes radial inward expansion. As a result, the configuration of the spirally wound electrodes is distorted, stack pressure is compromised and the axial void for gas recombination is lost.
It is a primary object of the present invention to overcome drawbacks of conventional winding methods, including those enumerated above, for producing spirally wound cell packs for electrolytic cells. It is a further object to produce a relatively small diameter cell pack using a small diameter solid mandrel yet produce high stack pressure among the components of the cell pack. It is another object to allow the use of an open face winding machine which will accommodate easy maintenance. It is still another object of the invention to maintain a small axial void within the electrolytic cell even after electrolyte has been added to the cell or cell operation has commenced.
These and other objects are provided for by the present invention as explained in more detail hereafter.